1. Field of the Invention
The present invention generally relates to laser-induced fluorescence and, more particularly, to fiber-coupled laser-induced fluorescence systems.
2. Description of the Related Art
The development of laser-induced fluorescence (LIF) as a combustion-diagnostic technique has permitted spatially resolved species-concentration and temperature measurements in reacting flows. LIF has particular advantages over other laser-diagnostic techniques because of its high detection sensitivity and excellent spatial resolution, which permit accurate measurements of minor species that play a critical role in chemical-kinetics mechanisms in reacting flows. Typical examples of such minor species are OH, CH, NH, and CN radicals as well as atomic species such as H and O. LIF also provides a straightforward, single-beam diagnostic approach that is experimentally less complicated than other Raman-based and wave-mixing techniques that require multiple laser beams for excitation. Furthermore, LIF can easily be extended to two-dimensional measurements using planar LIF (PLIF). Two-line OH-LIF has been demonstrated for thermometry in reacting flows. PLIF of tracer molecules has been used for mixture-fraction imaging and mixing studies in nitrogen-helium flows, where a non-fluorescing fuel is doped with a fluorescing tracer having ultraviolet (UV) excitation bands. Also, PLIF of seeded NO has been employed for pressure, temperature, and velocity measurements in supersonic and hypersonic flows. LIF has also been used to detect hydrocarbon species such as benzene, toluene, xylene, and ethylbenzene as well as polycyclic aromatic hydrocarbons (PAHs). A common spectroscopic feature of all of the aforementioned species is that their molecular transitions from the ground electronic state to the first excited state lie in the UV wavelength regime (200-450 nm), which requires that any laser-based spectroscopic technique must utilize UV lasers for excitation.
Traditional LIF techniques, which were developed to aid the understanding of fundamental combustion chemistry and dynamics in well-controlled laboratory flames, face a stiff challenge when implemented in practical combustion devices such as combustors and afterburners in practical gas-turbine engines. These harsh combustion environments are often associated with 1) extreme heat and vibrations, 2) unconditioned humidity and large thermal gradients, and 3) little or no optical access, which severely affects the free-standing optics used in traditional LIF measurements. These difficulties may be resolved by transmitting the required laser energy through optical fibers to a test section, with the laser system and detection hardware being located in an adjacent climate-controlled room. In particular, a fiber-coupled UV-LIF system would 1) reduce the need for standing optics in the test-cell environment, 2) ease the alignment of multiple laser beams, providing flexibility when needed and the ability to access non-windowed test sections, 3) isolate the high-power laser system from harsh environments, and 4) provide safe, guided, and confined laser delivery.
For long distance delivery of high-power laser pulses, the optimum fiber is fused-silica, solid-core fiber because absorption and bending loss are minimal. Some fiber-coupled UV-LIF systems have been developed using such a fiber for detection of petroleum products and biological samples in condensed phases. These systems have shown that the required pulse energy for LIF in condensed phases is considerably below the damage threshold of the fiber. However, in gas-phase media, because of the lower molecular densities, the effective optical depth is reduced by several orders of magnitude and, hence, the laser pulse energy required for LIF signal generation is at least one order of magnitude higher than that required for the condensed phase. Recently, UV-LIF of atmospheric OH and HO2 has been achieved through the use of multi-pass white cells for weak LIF signal enhancement. But, such a system has limited application in combustion and non-reacting flow diagnostics where measurements need to be performed without perturbing the system under study. A primary challenge in the development of a high-temperature, gas-phase, fiber-coupled UV-LIF system involves the delivery of sufficient laser energy through the fibers. Additional difficulties arise when the excitation laser beam is in the UV regime, where the transmission characteristics of UV laser pulses in silica fibers (e.g., transmission efficiency) degrade because of a solarization effect. Solarization is caused by the absorption of UV energy by silica material and results in a change in the transmission properties caused by the formation of defect centers that further reduce transmission and generate new wavelengths.
Accordingly, as a result of the technical barriers described above, there is a need in the art for a fiber-coupled UV-LIF system for gas-phase reacting-flow applications.